SPECTRAL REFLECTANCE OF SEA FOAM IN THE VISIBLE AND NEAR-INFRARED - IN-SITU MEASUREMENTS AND REMOTE-SENSING IMPLICATIONS

The spectral reflectance of sea foam was measured at the Scripps Insti tution of Oceanography Pier, La Jolla, California, by viewing the sea surface radiometrically in a region of breaking waves. Foam reflectanc e was found to decrease substantially with wavelength in the near-infr ared, contrary to the findings of previous studies, theoretical as wel l as experimental. Values in the visible (0.44 mu m) were reduced by t ypically 40% at 0.85 mu m, 50% at 1.02 mu m, and 85% at 1.65 mu m. The spectral effect was explained by the nature of the foam, which is com posed of large bubbles of air separated by a thin layer of water (foam stricto sensu) and of bubbles of air injected in the underlayer. The presence of bubbles in the underlayer enhances water absorption and th us reduces reflectance in the near-infrared. For ocean color remote se nsing, affected by the presence of foam and aerosols, the consequences of neglecting the spectral dependence of foam are dramatic. With only a small amount of foam, in the presence of aerosols or not and thus i rrespective of aerosol type, the errors in the retrieved water reflect ance at 0.44 mu m are above 0.01, which does not meet the accuracy goa l of 0.001 for biological applications. Since under normal conditions the effect of foam may have the same magnitude as the effect of aeroso ls, atmospheric corrections will be inaccurate (and useless) in many c ases, even taking into account the spectral dependence of the foam ref lectance. Space observations potentially contaminated by an effective foam reflectance (product of reflectance and fractional coverage) abov e 0.001, i.e., corresponding to wind speeds above 8 m s(-1), should be eliminated systematically. Utilization of near-infrared wavelengths a bove 0.9 mu m for atmospheric corrections of ocean color, possible wit h the moderate-resolution imaging spectrometer (MODIS), would aggravat e the problem. The measurements also indicated that foam significantly affects the retrieval of aerosol turbidity at 0.85 and 1.02 mu m for wind speeds above 10 m s(-1) but impacts minimally turbidity estimates at 1.65 mu m. Over the oceans the spectral range above 1 mu m is defi nitely recommended for remote sensing of tropospheric aerosol load and type from space.